1. Field of the Invention
This invention relates to a countermeasure for accidents on an electric circuit substrate, and particularly to the prevention of the production of an abnormal heavy current when a short-circuiting accident between circuit substrate patterns occurs. The accident may occur due to a solder ball or the like formed during the mounting of an electric element on an electric circuit substrate.
2. Related Background Art
When in various electric apparatuses, patterns on a substrate are short-circuited due to a solder ball or the like formed during mounting operations, various abnormal states occur. An example will hereinafter be described with reference to FIG. 3 of the accompanying drawings.
FIG. 3 is a circuit diagram of a DC/DC converter driven by a battery. In FIG. 3, the reference numeral 100 designates a lithium battery, the reference numeral 101 denotes a control circuit, the reference numeral 102 designates a coil, the reference numeral 103 denotes a switching FET transistor, the reference numeral 104 designates a diode, and the reference numerals 106 and 107 denote resistors for dividing the output voltage of the DC/DC converter, and the divided voltages are fed back to the control circuit 101. The reference numeral 105 designates a smoothing capacitor.
This DC/DC converter circuit transfers electrical energy stored in the coil to the capacitor 105 in conformity with the repetitive switching operation of the FET transistor 103 to thereby obtain a boosted output.
In the usual operation, the divided voltages of the resistors 106 and 107 of the output voltage of the DC/DC converter are inputted to the control circuit 101, which controls the duty width of the switching of FET transistor 103 so that the divided voltages may assume predetermined values, whereby a predetermined DC/DC output voltage can be obtained.
In such a circuit, for example, consider a case where, due to a solder ball or the like formed during the mounting of an electric element, the gate terminal of the FET transistor 103 in FIG. 3 and the positive pole of the battery 100 are short-circuited, as indicated by a dotted line a.
In this case, the gate of the FET transistor 103 is deeply biased by the voltage of the battery, and therefore, the FET transistor 103 becomes completely turned on. Consequently, an electric current determined by the resistance of the coil 102 and the ON resistance of the transistor 103 flows from the battery 100. However, the two resistance values are 1 ohm or less, and thus, a heavy current of several amperes flows out of the battery 100. If such a state lasts long, the FET transistor 103 may generate heat, and further may emit smoke. Particularly, when a transistor of a surface-mounted type having a low heat capacity is used as the FET transistor, this riskiness is high.
In order to prevent such an accident, it is conceivable to connect a fuse in series with the battery 100 and for this fuse to be cut off when the above-mentioned abnormal heavy current flows, to thereby prevent such an accident. But as a realistic problem, the setting of this fuse is difficult for the following reason.
In the case of the load of an actuator or the like, a considerable electric current flows from the output of the DC/DC converter even in a normal state, and a greater electric current flows from the battery 100. In this case, the fuse must not be cut off. On the other hand, for a heavy current in an abnormal state due to an accident, the fuse must be reliably cut off. The value of the heavy current in such an abnormal state due to an accident is considerably changed by the irregularity of the remaining capacity of the battery 100 and of the ON resistance value of the FET transistor 103. Consequently, the setting of a fuse which is not cut off for a heavy current in a normal state and is reliably cut off for a heavy current in an abnormal state due to an accident is often difficult.